Add validate_cart hook

src/pretix/base/signals.py

@@ -94,6 +94,19 @@ subclass of pretix.base.exporter.BaseExporter
 As with all event-plugin signals, the ``sender`` keyword argument will contain the event.
 """
 
+validate_cart = EventPluginSignal(
+    providing_args=["positions", "requested_add", "requested_delete"]
+)
+"""
+This signal is sent out every time a cart is about to be changed. It includes an iterable
+with the current CartPosition objects as well as lists of dictionaries of the cart items
+that the user wants to add. Those dictionaries can contain the keys ``item``, ``variation``,
+``count``, ``price`` and ``voucher``. The response of receivers will be ignored, but you can
+raise an OrderError with an appropriate exception message.
+
+As with all event-plugin signals, the ``sender`` keyword argument will contain the event.
+"""
+
 order_placed = EventPluginSignal(
     providing_args=["order"]
 )